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<a name="Anchored-Addresses"></a>
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<a name="Anchored-Addresses-1"></a>
<h3 class="section">18.14 Anchored Addresses</h3>
<a name="index-anchored-addresses"></a>
<a name="index-_002dfsection_002danchors-1"></a>

<p>GCC usually addresses every static object as a separate entity.
For example, if we have:
</p>
<div class="smallexample">
<pre class="smallexample">static int a, b, c;
int foo (void) { return a + b + c; }
</pre></div>

<p>the code for <code>foo</code> will usually calculate three separate symbolic
addresses: those of <code>a</code>, <code>b</code> and <code>c</code>.  On some targets,
it would be better to calculate just one symbolic address and access
the three variables relative to it.  The equivalent pseudocode would
be something like:
</p>
<div class="smallexample">
<pre class="smallexample">int foo (void)
{
  register int *xr = &amp;x;
  return xr[&amp;a - &amp;x] + xr[&amp;b - &amp;x] + xr[&amp;c - &amp;x];
}
</pre></div>

<p>(which isn&rsquo;t valid C).  We refer to shared addresses like <code>x</code> as
&ldquo;section anchors&rdquo;.  Their use is controlled by <samp>-fsection-anchors</samp>.
</p>
<p>The hooks below describe the target properties that GCC needs to know
in order to make effective use of section anchors.  It won&rsquo;t use
section anchors at all unless either <code>TARGET_MIN_ANCHOR_OFFSET</code>
or <code>TARGET_MAX_ANCHOR_OFFSET</code> is set to a nonzero value.
</p>
<dl>
<dt><a name="index-TARGET_005fMIN_005fANCHOR_005fOFFSET"></a>Target Hook: <em>HOST_WIDE_INT</em> <strong>TARGET_MIN_ANCHOR_OFFSET</strong></dt>
<dd><p>The minimum offset that should be applied to a section anchor.
On most targets, it should be the smallest offset that can be
applied to a base register while still giving a legitimate address
for every mode.  The default value is 0.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fMAX_005fANCHOR_005fOFFSET"></a>Target Hook: <em>HOST_WIDE_INT</em> <strong>TARGET_MAX_ANCHOR_OFFSET</strong></dt>
<dd><p>Like <code>TARGET_MIN_ANCHOR_OFFSET</code>, but the maximum (inclusive)
offset that should be applied to section anchors.  The default
value is 0.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fASM_005fOUTPUT_005fANCHOR"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_OUTPUT_ANCHOR</strong> <em>(rtx <var>x</var>)</em></dt>
<dd><p>Write the assembly code to define section anchor <var>x</var>, which is a
<code>SYMBOL_REF</code> for which &lsquo;<samp>SYMBOL_REF_ANCHOR_P (<var>x</var>)</samp>&rsquo; is true.
The hook is called with the assembly output position set to the beginning
of <code>SYMBOL_REF_BLOCK (<var>x</var>)</code>.
</p>
<p>If <code>ASM_OUTPUT_DEF</code> is available, the hook&rsquo;s default definition uses
it to define the symbol as &lsquo;<samp>. + SYMBOL_REF_BLOCK_OFFSET (<var>x</var>)</samp>&rsquo;.
If <code>ASM_OUTPUT_DEF</code> is not available, the hook&rsquo;s default definition
is <code>NULL</code>, which disables the use of section anchors altogether.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fUSE_005fANCHORS_005fFOR_005fSYMBOL_005fP"></a>Target Hook: <em>bool</em> <strong>TARGET_USE_ANCHORS_FOR_SYMBOL_P</strong> <em>(const_rtx <var>x</var>)</em></dt>
<dd><p>Return true if GCC should attempt to use anchors to access <code>SYMBOL_REF</code>
<var>x</var>.  You can assume &lsquo;<samp>SYMBOL_REF_HAS_BLOCK_INFO_P (<var>x</var>)</samp>&rsquo; and
&lsquo;<samp>!SYMBOL_REF_ANCHOR_P (<var>x</var>)</samp>&rsquo;.
</p>
<p>The default version is correct for most targets, but you might need to
intercept this hook to handle things like target-specific attributes
or target-specific sections.
</p></dd></dl>

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